Optical transmission/reception module

ABSTRACT

An optical transmission/reception module includes a substrate, a light-receiving element provided on said substrate, a light source provided on the substrate, an optical waveguide that is formed on the substrate and has a turnaround point, and a filter positioned between the optical waveguide and the light-receiving element. The optical waveguide is optically connected to the light source at one end and is optically connected to the light-receiving element at the turnaround point. There is a stray-light elimination section in the substrate between the light-receiving element and the aforementioned one end of the optical waveguide.

TECHNICAL FIELD

The present invention relates to an optical transmission/receptionmodule, and in particular, relates to an optical transmission/receptionmodule provided with a configuration for reducing stray light.

BACKGROUND ART

A fiber optic communication system in widespread use includes mainly atransmitter, an optical fiber corresponding to a transmission line, anda receiver.

At a point of connection between an optical waveguide type transmitterand an optic fiber, an optical signal transformed from an electricalsignal is inputted from an optical waveguide within the transmitter tothe optical fiber. The optical signal inputted to the optical fiber isthen transmitted by the optical fiber. At a point of connection betweenan optical waveguide type receiver and the optic fiber, the transmittedoptical signal is inputted to an optical waveguide of the receiver. Theoptical signal inputted to the optical waveguide is then inputted to alight receiving element such as a photo diode (PD), where the opticalsignal is transformed to an electrical signal.

In an optical transmission/reception module into which theabove-described receiver and transmitter are modularized together, aplurality of optical devices are arranged on an optical waveguide withinthe module. The plurality of optical devices include a laser diode (LD)corresponding to a light source, a PD corresponding to a light receivingelement, and the like.

In an optical transmission/reception module provided with such aconfiguration, out of an optical signal emitted from the light source, alight component not having been coupled into the optical waveguidebecomes “stray light”. The stray light propagates outside or inside thesubstrate of the optical waveguide. In particular, the stray lightpropagating inside the substrate becomes a cause of increasing noise inthe signal, as a result of its reentering the optical waveguide orentering the light receiving element connected to the optical waveguide.

In relation to this problem, technologies described in the followingPatent Literature 1 to Patent Literature 3 have been known as methodsfor eliminating stray light generated as described above.

FIG. 6 is a diagram showing a cross section of an optical modulatordescribed in Patent Literature 1. In Patent Literature 1, the opticalmodulator is described as one in which an optical waveguide 101 isformed on the top or bottom surface of a thin plate 102 and a straylight elimination means (light absorption section 103 in FIG. 6) isarranged within or close to the thin plate 102.

FIG. 7 is a diagram showing an optical IC (integrated circuit) describedin Patent Literature 2. In the optical IC of FIG. 7, an opticalwaveguide 101 is formed on the top surface of a substrate 104, and agroove (triangular groove 105) running approximately in parallel withthe optical waveguide 101 is formed at the bottom surface of thesubstrate 104 in a manner to be approximately opposed to the opticalwaveguide 101. Surfaces constituting the groove (triangular groove 105)are not perpendicular or parallel to the bottom surface of the substrate104. Patent Literature 3 describes an optical device provided with astray light reflection means for guiding stray light outside a thinplate on which an optical waveguide is formed.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No.2006-276518

Patent Literature 2: Japanese Patent Application Laid-Open No.2001-264553

Patent Literature 3: Japanese Patent Application Laid-Open No.2007-272121

SUMMARY OF INVENTION Technical Problem

In a case where a light source for transmission and a light receivingelement for reception are arranged on the same substrate of an opticaltransmission/reception module, it is possible that stray light generatedat a time of light emission of the light source enters the lightreceiving element after propagating inside the substrate. However, to anoptical transmission/reception module having such a configuration, thetechnologies for eliminating stray light of Patent Literature 1 toPatent Literature 3 cannot be applied, for the following reasons

The stray light elimination means in Patent Literature 1 (in FIG. 6, thelight absorption unit 103) is mainly arranged along the opticalwaveguide 101. Accordingly, the stray light elimination means describedin Patent Literature 1 can effectively eliminate only stray light withinthe thin plate 102, on which the optical waveguide 101 is formed, andstray light traveling in a direction perpendicular to the longitudinaldirection of the optical waveguide 101.

The groove in Patent Literature 2 (in FIG. 7, the triangular groove 105)also is arranged along the optical waveguide 101, and it reflects astray light component traveling perpendicularly to the longitudinaldirection of the optical waveguide 101, among the stray lightcomponents. Further, the stray light reflection means described inPatent Literature 3 also is arranged along the longitudinal direction ofthe optical waveguide. Accordingly, also the configurations described inPatent Literature 2 and Patent Literature 3 can effectively eliminateonly stray light traveling in a direction perpendicular to thelongitudinal direction of the optical waveguide.

For these reasons, in an optical transmission/reception module havingthe above-described configuration, even when the configurationsdescribed in Patent Literature 1 to Patent Literature 3 are applied toit, it is still possible that stray light generated at a time ofemission of an optical signal for transmission from the light sourceenters the light receiving element after propagating inside thesubstrate. As a result, in a case of Full-Duplex operation wheretransmission and reception are simultaneously performed, it is likelythat noise is added to a signal received by the light receiving element,and accordingly, the reception sensitivity of the light receivingelement is deteriorated.

The objective of the present invention is to solve the above-describedproblem and accordingly to provide an optical transmission/receptionmodule which reduces the deterioration of the reception sensitivity ofthe light receiving element.

Solution to Problem

An optical transmission/reception module of the present inventioncomprises a substrate, a light receiving element provided on thesubstrate, a light source provided on the substrate, an opticalwaveguide formed on the substrate and having a turnaround point, and afilter arranged between the optical waveguide and the light receivingelement, wherein the optical waveguide is optically connected with thelight source at its one end and with the light receiving element at theturnaround point, and a stray light elimination section is provided in apart of the substrate between the light receiving element and the oneend of the optical waveguide.

Advantageous Effect of Invention

The optical transmission/reception module of the present invention canreduce deterioration of the reception sensitivity of the light receivingelement.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A A top view of an optical transmission/reception module in afirst exemplary embodiment of the present invention

FIG. 1B A side view of the optical transmission/reception module in thefirst exemplary embodiment of the present invention

FIG. 2A A bottom view of an optical transmission/reception module inwhich another type of stray light elimination section is adopted, in thefirst exemplary embodiment of the present invention

FIG. 2B A side view of the optical transmission/reception module inwhich another type of stray light elimination section is adopted, in thefirst exemplary embodiment of the present invention

FIG. 3A A bottom view of an optical transmission/reception module inwhich still another type of stray light elimination section is adopted,in the first exemplary embodiment of the present invention

FIG. 3B A side view of the optical transmission/reception module inwhich still another type of stray light elimination section is adopted,in the first exemplary embodiment of the present invention

FIG. 4A A top view showing an example where an opticaltransmission/reception module is covered with a light absorptionmaterial, in the first exemplary embodiment of the present invention

FIG. 4B A side view showing the example where an opticaltransmission/reception module is covered with a light absorptionmaterial, in the first exemplary embodiment of the present invention

FIG. 5A A top view showing an example of an opticaltransmission/reception module of the present invention

FIG. 5B A side view showing the example of an opticaltransmission/reception module of the present invention

FIG. 6 A diagram showing a cross section of an optical modulatordescribed in Patent Literature 1

FIG. 7 A diagram showing an optical IC described in Patent Literature 2

DESCRIPTION OF EMBODIMENTS First Exemplary Embodiment

Hereinafter, an exemplary embodiment of the present invention will bedescribed using drawings. However, the exemplary embodiment does notlimit the scope of the present invention.

[Description of Configuration]

FIGS. 1A and 1B are a top view and a side view, respectively, of anoptical transmission/reception module 200 in the present exemplaryembodiment. The optical transmission/reception module 200 of the firstexemplary embodiment comprises a substrate 1, an optical waveguide 2, alight receiving element 3, a stray light elimination section 5, a lightsource 6 and a filter 7.

As shown in FIG. 1A, the optical waveguide 2 formed on the substrate 1has a turnaround point 4 a, one end 4 b and the other end 4 c. The lightreceiving element 3 is arranged on the substrate 1 and is opticallyconnected with the optical waveguide 2 at the turnaround point 4 a. Thelight receiving element 3 receives light having been transmitted throughthe optical waveguide 2, at the turnaround point 4 a via the filter 7.The light source 6 is arranged on the substrate 1. The light source 6 isoptically connected with the optical waveguide 2 at the one end 4 b.Emitted light from the light source 6 is coupled into the opticalwaveguide 2 at the one end 4 b and then propagates in the opticalwaveguide 2. At the other end 4 c, the emitted light having propagatedin the optical waveguide 2 is coupled into an optical transmission lineoutside the module, and then is transmitted in the optical transmissionline. The filter 7 is arranged between the turnaround point 4 a of theoptical waveguide 2 and the light receiving element 3. The filter 7passes light having entered the optical waveguide 2 from the opticaltransmission line outside the module. On the other hand, the filter 7totally reflects emitted light from the light source 6.

Of the optical waveguide 2, a portion ranging from the filter 7 to theother end 4 c is referred to as an optical waveguide 2 c, and that fromthe one end 4 b to the filter 7 as an optical waveguide 2 d. Theincident light having passed through the filter 7 is received by thelight receiving element 3. On the other hand, the emitted light havingbeen totally reflected by the filter 7 propagates through the opticalwaveguide 2 c, and then at the other end 4 c, it becomes incident on theoptical transmission line or the like outside the module.

Further, as shown in FIG. 1B, a stray light elimination section 5 isarranged in a part of the substrate 1 between the light receivingelement 3 and the one end 4 b of the optical waveguide 2.

In the following, specific configurations and materials which can beadopted in the optical transmission/reception module 200 will bedescribed.

As the substrate 1, a Si substrate 1 a can be used, as will be describedlater using FIGS. 5A and 5B.

The optical waveguide 2 may be configured with a planer lightwavecircuit (PLC) in which a core corresponding to the optical waveguide 2is embedded in a clad. The optical waveguide 2 illustrated in FIGS. 1Aand 1B may be configured with a PLC whose clad material is SiO₂. As willbe described later using FIGS. 5A and 5B, as materials of the PLC, anSiO₂ layer 2 a may be used for the clad, and SiO₂—GeO₂ glass for thecore. For example, a pattern of the core of SiO₂—GeO₂ glass is formed onthe SiO₂ layer 2 a by photolithography and known process technology.Besides those described above, any other known materials and fabricationmethods for forming the optical waveguide 2 may also be adopted.

As the light receiving element 3, a PD or an APD (avalanche photodiode)may be used. Any other known optical sensor also may be used as thelight receiving element 3.

As shown in FIG. 1B, the stray light elimination section 5 is provided,as a groove 5 a, in a part of the substrate 1 between the lightreceiving element 3 and the one end 4 b. The stray light eliminationsection 5 cuts off stray light propagating inside the substrate 1. Thegroove 5 a may be formed to be approximately perpendicular to thelongitudinal direction of the optical waveguide 2 and to extend over thewhole width of the substrate 1. By a surface of the groove 5 a, which isapproximately perpendicular to the substrate 1, stray light having acomponent travelling in the longitudinal direction of the opticalwaveguide 2 is reflected. Further, by filling the groove 5 a with alight absorption material for absorbing stray light, a high refractiveindex material for refracting stray light in a manner to prevent thestray light from being incident on the light receiving element 3, or thelike, a larger effect of cutting off stray light can be achieved. FIG.1B shows a case of filling the groove 5 a with a light absorptionmaterial, a high refractive index material or the like, as justdescribed above. However, the groove 5 a may be filled with nothing andbe in a state of having only air within it.

FIGS. 2A and 2B are a bottom view and a side view, respectively, of anoptical transmission/reception module, in the first exemplaryembodiment, where another type of stray light elimination section isadopted. As shown in FIGS. 2A and 2B, a plurality of grooves 5 a may bearranged along the longitudinal direction of the optical waveguide 2.Thus arranging a plurality of grooves 5 a results in a situation where alarge number of surfaces exist on a path through which stray lightreaches the light receiving element 3, and it accordingly becomespossible to perform stray light elimination a plurality of times. As aresult, in such a case of arranging a plurality of grooves 5 a as thestray light elimination section 5, the effect of reducing the influenceof stray light, which is achieved by stray light's being reflected bythe surfaces of the grooves 5 a, becomes larger. Further, when thegrooves 5 are filled with a light absorption material, a larger effectof absorbing stray light is achieved.

FIGS. 3A and 3B are a bottom view and a side view, respectively, of anoptical transmission/reception module, in the first exemplaryembodiment, where still another type of stray light elimination sectionis adopted. In FIGS. 1A, 1B, 2A and 2B, the grooves 5 a are formed toextend over the whole width of the substrate 1. However, the grooveconfiguration may be such that grooves 5 a with a smaller length thanthe whole width of the substrate 1 are arranged alternately at one andthe other side surfaces of the substrate 1. By thus providing thegrooves 5 a in a manner to be arranged alternately, prevention ofstrength decrease of the substrate 1 can be achieved simultaneously withstray light reduction.

Further, when members such as the grooves 5 a are arranged alternately,it is effective, for suppressing the influence of stray light andpreventing strength decrease of the substrate 1, to arrange all of thegrooves 5 a alternately, as illustrated in FIGS. 3A and 3B. However, ifat least one pair of alternately arranged grooves 5 a is present, theeffect of suppressing the influence of stray light and preventingstrength decrease of the substrate 1 is achieved. Further, a largereffect of cutting off stray light is achieved if the grooves 5 a, whichare arranged alternately at one and the other side surfaces of thesubstrate 1, are arranged in a manner to cooperatively constitute onesurface extending over the whole width of the substrate 1 when they areviewed from the one end 4 b of the optical waveguide 2, which is a pointof stray light generation, into the direction toward the light receivingelement 3.

Further, stray light may be scattered by making the surfacesconstituting the grooves 5 a have unevenness (including a roughsurface). Alternatively, stray light may be guided into a directionother than that toward the light receiving element 3, by arranging alight guiding section for guiding the stray light which has the sameconfiguration as that of the optical waveguide 2, as the stray lightelimination section 5. Also as the stray light elimination section 5,something capable of preventing stray light propagating inside thesubstrate 1 from reaching the light receiving element 3 may be adopted.Further, a plurality of ones of the above-described specific examples orknown examples of the stray light elimination section 5 may be combinedtogether.

The stray light elimination section 5 may be arranged only at a locationwhere stray light is highly likely to pass through. For example, thestray light elimination section 5 may be arranged only just beneath theoptical waveguide 2 d and around there. The stray light eliminationsection 5 does not necessarily need to have a rectangular parallelepipedshape as shown in FIG. 1A, and may be arranged in any shape at a heightand a location which make it possible to cut off stray light travelinginside the substrate 1.

Here, an LD 6 a may be used as the light source 6.

As the filter 7, a known optical demultiplexer, such as an optic fiberor a dielectric multilayer film filter functioning as a WDM (wavelengthdivision multiplexing) filter 7 a, may be adopted.

In the case of the optical transmission/reception module 200 having thearrangement shown in FIG. 1A, optical characteristics of the filter 7with respect to received optical signals and transmitted optical signals(whether to pass or totally reflect the signals) are as describedbefore. Alternatively, the filter 7 of another type having differentoptical characteristics may be adopted, on the basis of arrangements ofthe light receiving element 3 and the light source 6 with respect to asurface of the filter 7 on which the optical signals are to be incident.That is, the adopted filter 7 may be that of a type which makes receivedoptical signals be incident on the light receiving element 3 and makesoptical signals to transmit be incident on an optical transmission lineor the like outside the device but not incident on the light receivingelement 3.

FIGS. 4A and 4B are a top view and a side view, respectively, showing anexample in which the optical transmission/reception module in the firstexemplary embodiment is covered with a light absorption material. Aregion enclosed by a dashed-dotted line 5 c, in FIGS. 4A and 4B, shows aregion where a light absorption material 5 b is applied. A regionenclosed by a dashed-dotted line 9 a, in FIGS. 4A and 4B, shows a regionwhere a transparent resin 9 is applied.

As shown in FIGS. 4A and 4B, in addition to arranging the stray lightelimination section 5, a light absorption material 5 b may be applied ina manner to entirely cover the optical transmission/reception module200, in order to absorb stray light including that radiated outside thesubstrate 1.

Before the light absorption material 5 b is applied, the transparentresin 9 is applied between the one end 4 b of the optical waveguide 2and the light source 6, between the turnaround point 4 a of the opticalwaveguide 2 and the filter 7, and between the filter 7 and the lightreceiving element 3. The transparent resin 9 is further applied betweenthe optical transmission line outside the module and the other end 4 cof the optical waveguide 2, and also onto the light receiving element 3and the light source 6, before the light absorption material 5 b isapplied. The purpose of coating the above-described portions with thetransparent resin 9 is to prevent the light absorption material 5 b fromentering the optical path. The transparent resin 9 is applied such thatthe light receiving element 3 and the light source 6 are not opticallycoupled with each other through the transparent resin 9.

After the transparent resin 9 is applied, the opticaltransmission/reception module 200 is sealed such that its whole bodyincluding the transparent resin 9 is covered with the light absorptionmaterial 5 b. Because the transparent resin 9 is applied in a manner notto optically couple the light receiving element 3 with the light source6, stray light radiated outside around the light source 6 is absorbed bythe light absorption material 5 b covering the whole body. As a result,the stray light radiated outside around the light source 6 is notreceived by the light receiving element 3.

[Description of operation] Next, a description will be given ofoperation of the optical transmission/reception module 200, in thepresent exemplary embodiment, in transmission and reception.

(Operation in reception) In FIGS. 1A and 1B, an optical signal havingpropagated through the optical transmission line outside the moduleenters the optical waveguide 2 c from the other end 4 c, then propagatesin the optical waveguide 2 c, is thus guided to the light receivingelement 3 via the filter 7, and is then received by the light receivingelement 3. A component of emitted light from the light source 6 nothaving entered the optical waveguide 2 d at a time of the emittedlight's being incident on the optical waveguide 2 d at the one end 4 bbecomes stray light, and propagates inside the substrate 1. If the straylight reaches the light receiving element 3 in Full-Duplex operation,there occurs deterioration of the reception sensitivity, but the straylight component propagating inside the substrate 1 is cut off by thestray light elimination section 5 before the light receiving element 3.

(Operation in transmission) Emitted light from the light source 6 entersthe optical waveguide 2 d at the one end 4 b, then propagates in theoptical waveguide 2 d, and is guided by the filter 7 to the opticalwaveguide 2 c connecting with the optical transmission line or the likeoutside the module. The emitted light guided to the optical waveguide 2c enters the optical transmission line outside the module, and ispropagated in it. A component of the emitted light not having enteredthe optical waveguide 2 at a time of the emitted light's being outputtedfrom the light source 6 to the one end 4 b becomes stray light, andpropagates inside the substrate 1. If the stray light reaches the lightreceiving element 3 in Full-Duplex operation, there occurs deteriorationof the reception sensitivity, but the stray light component propagatinginside the substrate 1 is cut off by the stray light elimination section5 before the light receiving element 3.

[Description of Effect]

The optical transmission/reception module 200 corresponding to the firstexemplary embodiment of the present invention has the stray lightelimination section 5 in a part of the substrate 1 between the lightreceiving element 3 and the one end 4 b. Accordingly, the opticaltransmission/reception module 200 can cut off stray light generated atthe one end 4 b when emitted light from the light source 6 is incidenton the optical waveguide 2, which then propagates inside the substrate1, before the light receiving element 3. As a result, an opticalcross-talk between the light source 6 and the light receiving element 3is suppressed, and deterioration of the reception sensitivity inFull-Duplex operation can be reduced.

Example 1

[Description of Configuration]

FIGS. 5A and 5B are a top view and a side view, respectively, showing anexample of the optical transmission/reception module. As shown in FIGS.5A and 5B, an SiO₂ layer 2 a is formed on a Si substrate 1 a. Also onthe Si substrate 1 a, an LD 6 a corresponding to the light source, areception PD 3 a and a monitoring PD 3 b are mounted. As an opticaltransmission line outside the module, an optical fiber is mounted. Here,the monitoring PD 3 b is a monitoring unit which monitors light emittedfrom the surface (rear surface) of the LD 6 a on the opposite side toone end 4 b. As shown in FIGS. 5A and 5B, a recessed section 8 isprovided at a turnaround point 4 a of an optical waveguide 2 formed inthe SiO₂ layer 2 a on the Si substrate 1 a. In the recessed section 8, aWDM filter 7 a is installed at the turnaround point 4 a such that lighthaving propagated in the optical waveguide 2 from the one end 4 b isreflected there and then propagates in the waveguide 2 toward the otherend 4 c. Further, at the bottom surface of a part of the Si substrate 1a between the one end 4 b, which is optically connected with the LD 6 a,and the reception PD 3 a, a groove 5 a is formed over the whole width ofthe Si substrate 1 a. The groove 5 a at the bottom surface of the Sisubstrate 1 a is filled with a light absorption material 5 b.

[Description of Operation]

Emitted light from the front surface of the LD 6 a is coupled into andpropagates in the optical waveguide 2, then totally reflected by the WDMfilter 7 a, and is then transmitted propagating through the opticalwaveguide 2 and the optical fiber outside the optical waveguide 2, whichis connected with the optical waveguide 2 at the other end 4 c.Monitoring light emitted from the rear surface of the LD 6 a is receivedby the monitoring PD 3 b.

On the other hand, received light having a different wavelength fromthat of the emitted light enters the optical fiber outside the modulefrom a transmission line outside the module, and after propagatingthrough the optical fiber and the optical waveguide 2, it passes throughthe WDM filter 7 a and is then received by the reception PD 3 a. Thetransmission by the LD 6 a and the reception by the reception PD 3 a canbe performed simultaneously.

Out of the emitted light from the front and rear surfaces of the LD 6 a,a light component not having been coupled into the optical waveguide 2and that not having been received by the monitoring PD 3 b become straylight. The stray light propagates in a space outside the SiO₂ layer 2a/Si substrate 1 a, within the SiO₂ layer 2 a or within the Si substrate1 a.

There occurs deterioration of the reception sensitivity if the straylight reaches the reception PD 3 a, but in the present example, itsreaching the reception PD 3 a is prevented in the following ways. First,as shown in FIGS. 5A and 5B, the stray light component propagating inthe SiO₂ layer 2 a is cut off by covering entirely the cross section ofthe SiO₂ layer 2 a facing the reception PD 3 a with the WDM filter 7 a,which reflects the transmission wavelength, just before the reception PD3 a. Further, the stray light component propagating in the Si substrate1 is blocked from reaching the reception PD 3 a, at the groove 5 aprovided at the bottom surface and filled with the light absorptionmaterial 5 b, as a result of the stray light component's being reflectedby a surface of the groove 5 a or absorbed by the light absorptionmaterial 5 b packed in the groove 5 a. Further, also in the presentexample, the whole body of the optical transmission/reception module maybe covered with the light absorption material 5 b, similarly to in FIGS.4A and 4B. In the present case, as portions to which the transparentresin 9 is applied, that between the monitoring PD 3 b and the LD 6 andthat of the monitoring PD 3 b are added, besides the portionscorresponding to those described in the first exemplary embodiment. As aresult of the above-described ways, an optical crosstalk between the LD6 a and the reception PD 3 a is suppressed, and deterioration of thereception sensitivity in Full-Duplex operation can be reduced.

The exemplary embodiment of the present invention has been describedabove, with reference to the exemplary embodiment and the example.However, an aspect to which the present invention is applicable is notlimited to the exemplary embodiment described above. To theconfigurations and details of the present invention, various changeswhich can be understood by those skilled in the art may be made withinthe scope of the present invention.

The present application is based upon and claims the benefit of priorityfrom Japanese Patent Application No. 2013-149557, filed on Jul. 18,2013, the disclosure of which is incorporated herein in its entirety byreference.

REFERENCE SIGNS LIST

-   -   1 substrate    -   1 a Si substrate    -   2, 2 c, 2 d optical waveguide    -   2 a SiO₂ layer    -   3 light receiving element    -   3 a reception PD    -   3 b monitoring PD    -   4 a turnaround point    -   4 b one end    -   4 c the other end    -   5 stray light elimination section    -   5 a groove    -   5 b light absorption material    -   6 light source    -   6 a LD    -   7 filter    -   7 a WDM filter    -   8 recessed section    -   9 transparent resin    -   101 optical waveguide    -   102 thin plate    -   103 light absorption section    -   104 substrate    -   105 triangular groove    -   200 optical transmission/reception module

The invention claimed is:
 1. An optical transmission/reception modulecomprising: a substrate; a light receiving element provided on thesubstrate; a light source provided on the substrate; an opticalwaveguide formed on the substrate and having a turnaround point; and afilter arranged between the optical waveguide and the light receivingelement, wherein: the optical waveguide is optically connected with thelight source at its one end and with the light receiving element at theturnaround point; a stray light elimination section is provided in apart of the substrate between the light receiving element and the oneend of the optical waveguide; and wherein the stray light eliminationsection comprises one in which grooves are arranged alternately at oneand the other side surfaces of the substrate.
 2. The opticaltransmission/reception module according to claim 1, wherein the straylight elimination section is arranged in a part of the substrate betweenthe light receiving element and the one end of the optical waveguide, ina manner to cross over the optical waveguide.
 3. The opticaltransmission/reception module according to claim 2, wherein the straylight elimination section is arranged in a part of the substrate betweenthe light receiving element and the one end of the optical waveguide andthat between the light receiving element and the other end of theoptical waveguide, in a manner to cross over the optical waveguide. 4.The optical transmission/reception module according to claim 2, whereinthe stray light elimination section is arranged to extend over the wholewidth of the substrate.
 5. The optical transmission/reception moduleaccording to claim 2, wherein the stray light elimination sectioncomprises a groove.
 6. The optical transmission/reception moduleaccording to claim 1, wherein the stray light elimination section isarranged in a part of the substrate between the light receiving elementand the one end of the optical waveguide and that between the lightreceiving element and the other end of the optical waveguide, in amanner to cross over the optical waveguide.
 7. The opticaltransmission/reception module according to claim 6, wherein the otherend is connected with an optical transmission line outside the module.8. The optical transmission/reception module according to claim 6,wherein the stray light elimination section is arranged to extend overthe whole width of the substrate.
 9. The optical transmission/receptionmodule according to claim 6, wherein the stray light elimination sectioncomprises a groove.
 10. The optical transmission/reception moduleaccording to claim 1, wherein the stray light elimination section isarranged to extend over the whole width of the substrate.
 11. Theoptical transmission/reception module according to claim 10, wherein thestray light elimination section comprises a groove.
 12. The opticaltransmission/reception module according to claim 1, wherein the groovesare each filled with a light absorption material or a high refractiveindex material.
 13. The optical transmission/reception module accordingto claim 1, wherein at least part of a top surface of the opticaltransmission/reception module is covered with a light absorptionmaterial.
 14. The optical transmission/reception module according toclaim 1 further comprising a monitoring unit for monitoring output ofthe light source.